1. Technical Field
The present invention relates to methods for the combustionless measurement of the quality of gaseous fuels fed to gas consumption devices, and particularly natural gas consumption devices, as well as apparatus for the carrying out of the different variations of the process.
2. Description of the Prior Art
The heating value of a substance is of significant interest because it forms one basis for determining the commercial value of that substance as a fuel. Methods for measuring the quality of gaseous fuels to ascertain the amount of heat available therefrom are already being used in practice for numerous purposes. Recently, interest in and need for such measurements have increased considerably for various reasons. In industrial heating processes, it is frequently necessary to feed a well defined amount of heat per unit of time to a furnace in order to obtain optimum results. In other cases it is desirable to optimize the consumption of fuel, i.e., to feed only the amount of heat actually required even if supplying a larger amount of heat does not adversely affect the process or product. For accounting purposes, billing on the basis of the amount of heat supplied has also been preferred to billing on a volume basis.
An extensive need has arisen, therefore, for measurements of the quality of a gaseous fuel. Unfortunately, gas quality measurement is complicated by the fact that combustion gases, and particularly natural gases, are typically distributed together notwithstanding separate origin, composition and properties that differ to a greater or lesser extent from each other. Since processes and apparatus have proven themselves for use with such distributed gases of different composition, the on-line measurement of the quality of the gas or the quantity of heat available therefrom has gained increasing importance for the industrial use of gas and for accounting purposes.
Known methods for evaluating gas quality are in most cases not readily adaptable to these uses due to technical reasons or because of cost considerations. For example, one conventional method for measuring heating value known in the art comprises combustion calorimetry. This process involves the burning of a partial stream of the combustible gas with an open flame or with a catalyst and measuring the heat produced. The necessity of burning a measured partial stream of the gas in order to determine its heating value, as known from experience, requires frequent maintenance of the apparatus, since a flame can change due to deposits of combustion residues or because a combustion catalyst gradually declines in effectiveness. The required accuracy of measurements which serve, for example, for billing purposes can only be obtained if these apparatus are operated under well-defined, controlled conditions, preferably in an air-conditioned chamber, which is obviously expensive.
Other known non-combustion methods for continuously analyzing a stream of gas include gas chromatography and mass spectrometry. Gas chromatography and mass spectrometry are techniques for separating and identifying each constituent of the gas and measuring the relative concentration thereof. Knowing the heating value of each constituent of a mixture, the total heating value may then be computed. Unfortunately, these methods require a large expenditure of measurement and control devices to implement.
The same techniques are also currently used to determine other parameters representative of the quality of a distributed fuel gas, such as density and percent concentration of inert gases therein. A principle use for gas density determination is in the operation of an orifice flow meter, while percent concentration of inert gases, for example, nitrogen, N.sub.2, carbon dioxide, CO.sub.2, and oxygen, O.sub.2, is used to determine gas pumping/transportation cost or for subsequent regulation of a combustion process.
To summarize, most, if not all, presently known techniques for determining the quality of a fuel gas, such as heating value, density or percent concentration of inert gases, have one or more drawbacks associated therewith, including: requiring trained personnel to operate, producing time delayed results, lacking repeatability, destroying the sample, being cumbersome or expensive to implement and lacking sufficient accuracy due to an inability to completely distinguish constituents. Therefore, there exists a genuine need in the art for a novel approach to the measurement of the quality of a fuel gas which is accurate, reliable and inexpensive in implementation.